1. Field of the Invention
The present invention relates to an optical component including a holder to which a graded-index rod lens used in the fields of microoptics and optical communications is fixedly mounted.
2. Description of the Related Art
A graded-index rod lens has a capability of refracting light by a refractive index distribution existing in the medium itself and, therefore, the rod lens is fundamentally a cylindrical lens having flat end faces. For this reason, the rod lens shows better geometric matching to other optical elements such as optical fibers compared with general lenses. This is advantageous in making an optical alignment. Furthermore, the medium itself has refractive power and so the focal distance can be varied by varying the lens length. In this way, the rod lens has many excellent characteristics. Consequently, rod lenses of this construction are widely used in the field of microoptics. Also, in the field of optical communications, they are widely recognized as very important optical devices.
In addition, the refractive power of this graded-index rod lens can be increased further by machining at least one end face of the lens into a spherical surface. Thus, a lens which is brighter and has a larger numerical aperture than a lens having flat end faces can be obtained (see Japanese Patent Publication No. JP 2002-182073A, for example).
Other lenses are often mounted to a holder (frame) as well as the aforementioned rod lens, in order that they be handled with ease and be assembled with ease when combined with other optical devices. This is known as mounting to a holder and the following characteristics are important.
(1) The positional accuracy of the lens with respect to the holder
(2) The adhesive strength of the lens to the holder
(3) Airtightness after the holder and lens have been bonded together
(4) Durability of the bonding
A method of mounting a rod lens and a semiconductor laser to a holder and integrating them to couple light emerging from the laser to an optical fiber is disclosed, for example, in Japanese Patent Publication JP H07-281062A. Since the diameter of the rod lens is normally as small as several millimeters or less, mounting to the holder is especially important. The holder is made from a metal or resin. The following methods are generally used to fixedly mount the lens to the holder:
(a) Adhesive bonding using metal solder
(b) Adhesive bonding using an organic adhesive
(c) Adhesive bonding using a low-melting-point glass
(d) Press fitting
An example of how a rod lens is mounted to a lens holder is shown in FIG. 8. The lens holder, indicated by numeral 130, is made of a metal and assumes a bell-shaped form. The holder has a top portion in which a through hole is formed. The rod lens, indicated by numeral 10, is inserted and fixed in this through hole. In this case, the rod lens 10 is fixedly mounted to the metallic lens holder 130 by low-melting-point glass 110.
However, machining end faces of rod lenses into spherical surfaces suffers from low productivity. Especially, where the diameter of a rod lens is reduced, it becomes more difficult to polish the end faces of the lens.
Furthermore, the above-described means (a)–(d) used in mounting the lens to the holder have the following problems.
In order to bond a glass lens using a metal solder, it is necessary to form a metal film on the outer surface of the lens by metal plating or vacuum film deposition. Furthermore, the holder needs to be plated with gold. Consequently, a large number of processing steps are required. This leads to an increase in the cost.
The method of mounting the lens with an organic adhesive is simple to achieve but the durability has a problem. In addition, if degassing from the resin occurs, depositions are produced on the surface of the lens or the semiconductor optical device is adversely affected.
Generally, bonding using low-melting-point glass is widely known as a method of bonding the window material of a cap (cover) portion of a light-emitting or photodetecting device and provides high durability. However, the low-melting-point glass that is not yet melted is solid and so a machining operation conforming to the profiles of the bonded portions of the holder and lens is necessary. In the example shown in FIG. 8, low-melting-point glass that has been machined into an annular form is used to surround the rod lens. In addition, a care must be paid to the matching among the low-melting-point glass, holder, and lens in terms of coefficient of thermal expansion. In this way, there are many limitations.
Mounting using press fitting is advantageous in that no adhesive medium is required. However, the lens and holder are required to have high dimensional accuracy for the press fitting. Another problem is that press fitting produces internal stress in the glass lens, deteriorating the polarization characteristics.